My invention relates to rectifiers and, more particularly, to two-terminal semiconductor rectifiers especially suitable for high-speed power-switching operation.
Rectifiers are electrical devices that are particularly adapted to rectifying current, that is converting alternating current to direct current. More specifically, rectifiers exhibit a very low resistance to current flow when forward-biased (i.e., anode biased more positive than cathode) and a very high resistance to current flow when reverse-biased (i.e., anode biased more negative than cathode).
One known form of rectifier is a semiconductor p-i-n diode, which typically comprises semiconductor layers arranged as P.sup.+ /N/N.sup.+. The "P.sup.+ " and "N.sup.+ " layers constitute semiconductor regions that are highly-doped with P-conductivity type dopant and with N-conductivity type dopant, respectively. The intermediate "N" layer is relatively lightly doped with N-conductivity type dopant so that it can support high reverse voltages without current conduction.
In operation of a p-i-n diode, a forward bias of typically 0.8 to 1.0 volts (for silicon devices) is required to initiate current conduction. This forward voltage drop of 0.8 to 1.0 volts undesirably results in a high level of waste heat generation during forward conduction; a rectifier with a lower forward voltage drop would thus be desirable so as to limit waste heat generation.
A p-i-n diode is a "bipolar" device in that current flow in the diode is due to current carriers of both types, that is, both holes and electrons. Relative to unipolar devices in which current flow is due to only hole or electron flow, bipolar devices are slow at turning off, since, after turn-off initiation in a bipolar device, there is a delay during which minority current carriers (i.e., holes in the "N" region of a typical p-i-n diode) recombine with electrons. The slower turn-off speed of bipolar devices make them less suitable than unipolar devices for high-speed switching applications.
A rectifier that was developed to provide a lower forward voltage drop and a faster turn-off speed than a p-i-n diode is the Schottky diode. In a typical Schottky diode, a Schottky barrier contact is formed between a first electrode and a first N-conductivity type layer of semiconductor material. The first layer has a dopant concentration per cubic centimeter below about 1.times.10.sup.17, at least for N-conductivity type silicon. A Schottky barrier contact exhibits a potential barrier to current flow and, like a p-i-n diode, must be forward biased to initiate current flow. If the first layer had a dopant concentration in excess of the foregoing value, an ohmic contact between the first electrode and first layer would result, which does not exhibit a potential barrier to current flow. In the foregoing Schottky diode, an ohmic contact is formed between a second N-conductivity type layer of more highly doped semiconductor material adjoining the first layer and a second electrode.
Although Schottky diodes exhibit lower forward voltage drops and faster turn-off speeds than p-i-n diodes, this is at the expense of exhibiting high reverse leakage currents, which increase significantly for increasing values of reverse voltage.
Accordingly, it is an object of my invention to provide a rectifier that attains a low forward voltage drop and fast turn-off speed without exhibiting a high level of reverse leakage current.
A further object of my invention is to provide a rectifier that is particularly suitable for high-speed power-switching applications.
A still further object of my invention is to provide a unipolar semiconductor rectifier of low forward voltage drop and low reverse leakage current that can be fabricated using conventional semiconductor processing techniques.
The foregoing objects are attained in a pinch rectifier which, in a preferred embodiment, includes an N.sup.+ substrate layer with an N.sup.- epitaxial layer grown thereon. Adjoining the upper portion of the N.sup.- epitaxial layer is a current pinch-off means embodied as a plurality of P.sup.+ region portions spaced from each other so as to define between adjacent P.sup.+ region portions respective conduction channel portions in the N.sup.- epitaxial layer. An anode is provided atop both the current pinch-off means and the conduction channel portions in the N.sup.- epitaxial layer. The anode forms an ohmic contact with the channel pinch-off means, but forms a Schottky barrier contact with the conduction channel portions in the N.sup.- epitaxial layer.
The current pinch-off means is effective to induce depletion region portions in the N.sup.- epitaxial layer extending therein from the P.sup.+ region portions, respectively. The P.sup.+ region portions of the current pinch-off means are spaced sufficiently near each other that the induced depletion region portions in the N.sup.- epitaxial layer merge together so as to pinch off the conductive channel portions upon biasing of the pinch rectifier with a sufficient reverse voltage. At reverse voltages of lower magnitude, the Schottky barrier contact between the anode and the N.sup.- epitaxial layer prevents current conduction by the pinch rectifier.
One further embodiment of my invention employs a conductive material for the current pinch-off means, rather than the above-described P.sup.+ region portions, which material forms a Schottky barrier contact with the N.sup.- epitaxial layer. A still further embodiment of my invention includes a highly-doped N conductivity type region sandwiched between the anode and the N.sup.- epitaxial layer so that the anode forms an ohmic contact to the N.sup.- epitaxial layer, rather than the Schottky barrier contact described above with respect to the preferred embodiment.